When using the newly fixed

tag on the lingering potion, the duration, in seconds, will be divided by a specific amount. These are the times I tested.

200 ticks (10 secs) changes to 40 ticks (2 secs) (time is divided by 5)
600 ticks (30 secs) changes to 140 ticks (7 secs) (time is divided by 4.286)
1200 ticks (1 min) changes to 300 ticks (15 secs) (time is divided by 4)
2400 ticks (2 mins) changes to 600 ticks (30 secs) (time is divided by 4)
3600 ticks (3 mins) changes to 900 ticks (45 secs) (time is divided by 4)
8400 ticks (7 mins) changes to 2100 ticks (1min 45secs) (time is divided by 4)

You may use this command to compare your results with mine. Execute the splash potion command first, then the lingering potion.
Splash Potion:

 /give @p splash_potion 1 0 {CustomPotionEffects:[{Id:25,Amplifier:1,Duration:8400}]} 

Lingering Potion:

 /give @p lingering_potion 1 0 {CustomPotionEffects:[{Id:25,Amplifier:1,Duration:8400}]}